I've read some non-Cisco documentation that EIGRP will not allow adjacencies to form when secondary addresses are used. This is incorrect, but there is one common error that can result if both addresses are not secondary. To fully prepare for the 642-901 BSCI exam, you should know about this error. Let's take a look at R2 and R3, which will be using secondary addresses to form an EIGRP adjacency across an ethernet segment.
Here's the partial output of show ip eigrp neighbor on R3:
R3#show ip eigrp neighbor
IP-EIGRP neighbors for process 100
H Address Interface
0 22.214.171.124 Et0
The adjacency has formed! Note the address is actually the primary IP address on the interface, even though we used the secondary network number in the EIGRP network command. Personally, I stay away from secondary network numbers if at all possible, but you should know that secondary IP addresses can be used to create EIGRP adjacencies.
What's the common error with using secondary addresses, you asked? It's when an address from the same subnet is the primary interface address on one neighbor and the secondary interface address on another. Let's say we had configured R2 and R3 as follows:
01:54:05: IP-EIGRP: Neighbor 126.96.36.199 not on common subnet for Ethernet0 (188.8.131.52 255.255.255.0)
Since we configured 184.108.40.206 as a primary interface address on R2, the EIGRP process is looking at the primary interface address on potential neighbors. R3's primary ethernet0 address is 220.127.116.11, so you get the "not on common subnet" error message - and what you don't get is an adjacency! That's something to be aware of on your 642-901 BSCI exam as well as when working with EIGRP in production networks.
About the Author:
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage , home of free CCNA and CCNP tutorials! Pass the CCNA exam with Chris Bryant!